The present invention relates to an image signal amplifier, especially used for pre-processing of an image signal in a real time image recognition circuit.
For example by taking a picture of someone's face by television camera and processing the image signal to recognize movement of the mouth or eyes, power on/off or power up/down of a certain electric device can be controlled by optically recognizing such movement. Japanese Patent Application No. 60(1985)-272793 shows an example of this kind of device.
In this kind of optical control device, however, a device may not obtain image information necessary to recognize such movement when differences in image density are not sufficient. When a taken picture image is too dark or light, an automatic gain control (AGC) circuit is used to adjust an amplitude of the image signal to an appropriate density in order to recognize an image. Although a whole picture image may be light enough, a sampling area may be too dark or light. For example, in a bright background when people put on a hat and it makes a shadow on the eyes as shown in FIG. 6a, the eye part has less contrast than the contrast of a whole picture. When a light is applied to a subject from the side or at a slant, a part of the subject, for example hair, may make a shadow so that a contrast within the sampling area is lessened. In these situations, the AGC circuit may not adjust a signal level sufficiently to permit recognition.
When a microprocessor is used as an adjusting circuit, contrast can be adjusted by emphasizing a density of an image signal which is converted into a digital signal by the microprocessor. However, even digital processing has some problems, as mentioned below, when it is used for a real time image recognition circuit.
a) It is less effective if the differences in the density of the sampling area are not sufficiently bigger than an error of the quantization digital quantities.
b) It requires a high speed A/D converter with multi-bit operation and a large frame memory to minimize an error of the quantization digital quantities.
c) It takes a long time to process a recognizing operation of a microprocessor when there are many bits of image information.
When an amplitude of an image signal is adjusted as a full scale to the brightest part of an image signal, if a level of the brightest part is 100, an average density of a sampling area is 5, and the difference in density is 1, an error of the quantization of digital quantities has to be less than 1% in order to sample a preferred area. In this example, a preferred sample area has a 20% contrast; however, the difference in density is 1% of the full scale. Therefore, recognition is difficult.
When there is a wide dynamic range of image density, digital quantities have to be quantized by a large number of bits even if an image contains sufficient contrast This causes the above mentioned problems b) and c). Furthermore, when a part of an image information is emphasized by digital processing, data is required to show how much emphasis has to be done, so that this cannot be used for an automatic image recognition process.